A DC-DC converter is a circuit that converts an unregulated DC supply into a well-regulated one. The converter can be a linear regulator or a switching one. Linear regulators produce a ripple-free output voltage, but a DC-DC linear regulator can be used only when the output DC level is lower than the input level (e.g. this is known as a Buck operation). They also suffer from poor power efficiency when the difference between the input DC level and the output DC level is large.
Switching regulators, on the other hand, can be used when the output DC level is higher, lower or equal to the input DC level. Switching regulators provide better power efficiencies, but they suffer from output voltage ripples. Switching regulators can use inductors or capacitors as their energy-storage elements. Switched-capacitor DC-DC converters are gaining importance recently because they can provide a fully-integrated solution, as compared to switched-inductor ones.
FIG. 1 shows a prior art fully integrated switched-capacitor DC-DC converter. The conversion ratio (the quotient of the output DC level over the input DC level) of a switched-capacitor DC-DC converter is determined by the capacitor array topology. To regulate the output DC voltage at a different level, the equivalent output resistance of the converter should be changed, resulting in a degraded power efficiency. The switched-capacitor DC-DC converter in FIG. 1 may use a multiple-capacitor-array topology to produce multiple conversion ratios in order to avoid this tradeoff.
The switched-capacitor DC-DC converter in FIG. 1, for example, may comprise a capacitor and switch array (101), which includes multiple capacitors. Such a capacitor and switch array (101) can achieve different configurations and conversion ratios. A gain selector (102) is responsible for selecting the appropriate conversion ratio based on the input DC level and the desired output DC level. A clock generator (103) is responsible for generating the required clock phases for the converter, and a feedback controller and voltage-controlled oscillator (VCO) (104) is responsible for regulating the output DC level based on the output load within a selected conversion ratio. The number of conversion ratios achievable are determined by the number of capacitors and switches in the array, this leads to an optimized power efficiency.
However, the overhead of array re-configuration on converter complexity and power consumption can be high and unnecessary for certain ranges of input and output DC levels. The output ripples in capacitor-limited converters can also be prohibitively large. Capacitor-limited converters also suffer from area constraints when capacitor stacking is needed for input and/or output DC levels larger than the capacitors reliability voltage rating.
Thus, while the prior art switching regulators have been useful, there is still a need for capacitor-limited converters for high input and/or output DC level ranges.